Detecting broadside acoustic signals with a fiber optical distributed acoustic sensing (DAS) assembly

ABSTRACT

A method of distributed acoustic sensing includes providing a fiber optic distributed acoustic sensing system having a cable. A straight optical fiber extends parallel to a longitudinal axis of the cable along the cable length. A helically wrapped optical fiber extends along the cable length. The method includes transmitting optical signals into and receiving backscattered signals out of the optical fibers consisting of a component of said optical signals which component has been backscattered from impurities or inhomogeneities in the optical fibers, observing changes in the backscattered signals caused by axial stretching and compressing of the optical fibers caused by an incident wave, comparing the backscattered signals of the straight optical fiber and the helically wrapped fiber, and determining, based on the comparing of the backscattered signals, a direction of wave propagation of the incident wave with respect to the fiber axis for detecting broadside waves and axial waves distinguishably.

RELATED CASES

This application is a divisional application claiming benefit of U.S.application Ser. No. 14/365,231, filed on 13 Jun. 2014, which is a U.S.national stage application of International application No.PCT/US2012/069464, filed on 13 Dec. 2012, which claims priority fromU.S. application Ser. No. 61/576,192, filed on 15 Dec. 2011, each ofwhich are incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The invention relates to fiber optic devices and in particular to afiber optical Distributed Acoustic Sensing (DAS) assembly that isadapted to sense the magnitude and direction of acoustic signals, andparticularly those signals that are travelling at an angle orsubstantially perpendicular to the device.

BACKGROUND OF THE INVENTION

Various attempts have been made to provide sensing capabilities in thecontext of petroleum exploration, production, and monitoring, withvarying degrees of success. Recently, these attempts have included theuse of fiber optic cables to detect acoustic energy. Because the cablestypically comprise optically conducting fiber containing a plurality ofbackscattering inhomogeneities along the length of the fiber, suchsystems allow the distributed measurement of optical path length changesalong an optical fiber by measuring backscattered light from a laserpulse input into the fiber. Because they allow distributed sensing, suchsystems may be referred to as “distributed acoustic sensing” or “DAS”systems. One use of DAS systems is in seismic applications, in whichseismic sources at known locations transmit acoustic signals into theformation, and/or passive seismic sources emit acoustic energy. Thesignals are received at seismic sensors after passing through and/orreflecting through the formation. The received signals can be processedto give information about the formation through which they passed. Thistechnology can be used to record a variety of seismic information.Another application is in the field of in-well applications and acousticfluid monitoring.

DAS systems typically detect backscattering of short (1-10 meter) laserpulses from impurities or inhomogeneities in the optical fiber. If fiberis deformed by an incident seismic wave then 1) the distance betweenimpurities changes and 2) the speed of the laser pulses changes. Both ofthese effects influence the backscattering process. By observing changesin the backscattered signal it is possible to reconstruct the seismicwave amplitude. The first of the above effects appears only if the fiberis stretched or compressed axially. The second effect is present in caseof axial as well as radial fiber deformations. The second effect is,however, several times weaker than the first. Moreover, radialdeformations of the fiber are significantly damped by materialssurrounding the fiber. As a result, a conventional DAS system with astraight fiber is mainly sensitive to seismic waves polarized along thecable axis, such as compression (P) waves propagating along the cable orshear (S) waves propagating perpendicular to the cable. The strength ofthe signal varies approximately as cos² θ, where θ is the angle betweenthe fiber axis and the direction of wave propagation (for P waves).Thus, while there exists a variety of commercially available DAS systemsthat have varying sensitivity, dynamic range, spatial resolution,linearity, etc., all of these systems are primarily sensitive to axialstrain. Acoustic signals travelling normal to the fiber axis maysometimes be referred to as “broadside” signals and, for P waves, resultin radial strain on the fiber. Thus, as the angle between direction oftravel of the acoustic signal and the fiber axis approaches 90°, DAScables become much less sensitive to the signal and may even fail todetect it.

Hence, it is desirable to provide an improved cable that is moresensitive to signals travelling normal to its axis and enablesdistinguishing radial strain from the axial strain. Sensitivity tobroadside waves is particularly important for seismic or microseismicapplications, with cables on the surface or downhole. In addition tobroadside sensitivity, it is also desirable to provide three-component(3C) sensing, from which the direction of travel of the sensed signalcan be determined.

SUMMARY OF THE INVENTION

The present invention provides an improved fiber optic cable system fordistributed acoustic sensing that is more sensitive to signalstravelling normal to its axis and is thus better able to distinguishradial strain from axial strain on the system. Acoustic signalstravelling normal to the cable axis may sometimes be referred to as“broadside” signals and result in radial strain on the fiber. Thepresent invention also provides three-component (3C) sensing, from whichthe direction of travel of the sensed signal can be determined.

According to some embodiments, a distributed fiber optic acousticsensing system comprises an elongated body having an outer surface, anoptical fiber disposed on the outer surface at a first predeterminedwrap angle, and light transmitting and receiving means opticallyconnected to the fiber for, respectively, transmitting an optical signalinto the fiber and receiving a backscattered component of the signal outof the fiber. The system may further include a second optical fiberdisposed on the outer surface at a second predetermined wrap angle. Thewrap angles may be measured with respect to a plane normal to the axisof the body and the first wrap angle may be 90° and the second wrapangle may be less than 45°.

The system may further include a third fiber disposed on the outersurface at a wrap angle between 90° and 45°. At least one of the fibersmay include Bragg gratings.

The body may have a circular cross-section or an ellipticalcross-section and may include a layer of swellable elastomer surroundingthe body.

A sensing rod may be disposed in the elongated body and may contain atleast one additional fiber. The additional fiber(s) may be substantiallystraight, helical, or sinusoidal.

The system may further include layer of swellable elastomer between thesensing rod and the elongate body. Additionally or alternatively, thesystem may include a first sheath layer on the outside of the body andcovering the fiber. The first sheath layer may have an oval externalcross-section. The elongate body may have a non-circular cross-sectionhaving a larger semi-axis and the first sheath layer may be configuredso that its larger semi-axis is perpendicular to the larger semi-axis ofthe elongate body.

The system may include a second optical fiber wrapped around the outsideof the first sheath layer. The first fiber and the second fiber maydefine different wrap angles. The system may include a second sheathlayer on the outside of the first sheath layer and covering the secondfiber. At least one of the sheath layers preferably comprises apolyamide or material having a similar elastic impedance.

Other embodiments of a distributed fiber optic acoustic sensing systemcomprise an elongate body having an outer surface that includes at leastone substantially flat face, a first optical fiber housed in the body,and light transmitting and receiving means optically connected to thefiber for transmitting an optical signal into the fiber and receiving abackscattered component of the signal out of the fiber. The body mayhave a polygonal or triangular cross-section. The first fiber may besinusoidal and the system may include a second sinusoidal fiber defininga plane perpendicular to the plane of the first fiber. The system mayinclude a third fiber, which may be substantially straight or helical,and may define a wrap angle with respect to a plane normal to the axisof the body. The wrap angle may be less than 45° or less than 30°.

In preferred embodiments, the substantially flat face may have a visualappearance that is different from the appearance of the rest of theouter surface.

Still other embodiments of the invention include a distributed fiberoptic acoustic sensing system comprising an inner tube, a layer ofswellable elastomer surrounding the tube, a tube of swellable elastomersurrounding the elastomer layer and defining an annulus therewith, andat least one sensor pad or strip disposed in the elastomer tube, eachsensor pad comprising a stiffener and at least one longitudinal fiberaffixed thereto or embedded therein. The system may include at leastfour sensor pads are disposed in the elastomer tube. At least oneoptical fiber may be housed in the inner tube.

The inner tube may comprise a steel tube and the elastomer layer and theelastomer tube may be configured such that when they swell the annulusdisappears. The elastomer layer is further configured such that when itswells without being constrained, its diameter exceeds a predeterminedvalue that is selected to correspond to the inner diameter of a hole inthe earth. The elastomer tube may be further configured such that whenit swells in a borehole, the sensor pad(s) is/are disposed at the outersurface of the elastomer tube. The longitudinal fiber in each sensor padmay be sinusoidal, and/or each sensor pad may include one sinusoidallongitudinal fiber and one straight longitudinal fiber. At least one ofthe optical fibers may contain Bragg gratings.

Still other embodiments of a distributed fiber optic acoustic sensingsystem for use on a surface comprise an inner tube housing at least oneoptical fiber, a body of protective material surrounding the tube, thebody having an outer surface that includes at least one substantiallyflat face, and at least one sensor pad or strip disposed in the body,the sensor pad comprising a stiffener and at least one longitudinalfiber affixed thereto or embedded therein. The at least one sensor padmay also include at least one sinusoidal fiber affixed thereto orembedded therein. The system may include at least two sensor pads thateach include at least one sinusoidal fiber affixed thereto or embeddedtherein, and the two sensor pads may be mutually perpendicular. At leastone sensor pad may be adjacent to the flat face.

The inner tube may also houses at least one electrical transmissionline. The system may further including an anchor that is configured tooverlie the body and includes at least one arm for anchoring the anchorand body to the surface. The arm may be straight or curved.

As used herein the phrases “propagating along the fiber” and“propagating perpendicular to the fiber,” when used in reference to anacoustic signal, will be understood to refer to P waves that arepolarized along their direction of propagation.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more detailed understanding of the invention, reference is made tothe accompanying drawings wherein:

FIG. 1 is a schematic side view of a cable constructed in accordancewith one embodiment of the invention;

FIG. 2 is a schematic end view of the embodiment of FIG. 1;

FIG. 3 is a schematic transverse cross-section of a cable constructed inaccordance with another embodiment of the invention;

FIG. 4 is a schematic axial cross-section of an optical sensing systemin accordance with the invention in a borehole;

FIG. 5 is another view of the system of FIG. 4 after swelling of aswellable layer;

FIG. 6 is another view of the system of FIG. 5 showing placement of asensing rod in the system;

FIG. 7 is another view of the system of FIG. 6 after swelling of asecond swellable layer;

FIG. 8 is a schematic illustration of an optical sensing system inaccordance with another embodiment;

FIG. 9 is a schematic illustration of an optical sensing system inaccordance with another embodiment;

FIG. 10 is a schematic end view of the system of FIG. 9;

FIG. 11 is an axial cross-section of an optical sensing system inaccordance with another embodiment;

FIG. 12 is a cross-section taken along the lines 12-12 of FIG. 11;

FIG. 13 is a cross-section taken along the lines 13-13 of FIG. 11;

FIG. 14 is a schematic axial cross-section of the system of FIG. 11 in aborehole;

FIG. 15 shows the system of FIG. 13 after swelling of two swellablelayers;

FIG. 16 is a schematic axial cross-section of an embodiment of theinvention configured for use on the earth's surface; and

FIGS. 17 and 18 are alternative embodiments of a support device for usewith the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

DAS Cable with Helically Wrapped Fibers for Improved BroadsideSensitivity

One aspect of the present invention comprises a DAS fiber helicallywrapped around a cable or mandrel for the purpose of providing improvedbroadside sensitivity. A helically wound fiber will always includeportions of the fiber that form relatively small angles with theincident wave, independently of the angle of incidence. Assuming thatthe cable and fiber are perfectly coupled to the formation, one candetermine the fiber angular sensitivity S by projecting the wave strainalong the fiber axis. This gives:

$S = {{\cos^{2}{\theta sin}^{2}\alpha} + \frac{\sin^{2}{\theta cos}^{2}\alpha}{2}}$where α is the wrapping angle, i.e. the angle between the fiber and aplane perpendicular to the cable or mandrel axis, and θ is the angle ofincidence with respect to the cable of mandrel axis.

FIGS. 1 and 2 are schematic side and end views, respectively, of anoptical sensing system 10 comprising a fiber 11 wrapped helically with awrap angle of α around a cable 12 having an axis 14. While not describedin detail herein, it will be understood that the optical sensing systemsdisclosed below are used in conjunction with optical light transmittingand receiving means that are connected to the fiber for transmitting anoptical signal into the fiber and receiving a backscattered component ofthe transmitted signal out of the fiber.

The case of a straight fiber corresponds to the wrapping angle α=90°. Ifthe wrapping angle is about α=35°, then the fiber sensitivity will notdepend on the angle θ and will be equal to S=⅓. As can be seen, ahelically wrapped fiber does not change the overall sensitivity of thesystem, but redistributes it in three spatial directions.

The above sensitivity S refers to unit fiber length. The length of thefiber L_(f) is equal to L_(f)=L_(c)/sin α, where L_(c) is the length ofthe cable along which the fiber is wrapped. The sensitivity of ahelically wrapped fiber per unit cable length is thus (1/sin α) timeshigher than the above value of S. Hence, by using a helically wrappedfiber, it is possible to not only increase the broadside sensitivity ofthe system, but also its overall sensitivity by packing more fiberlength in the same cable length compared with a straight fiber.Alternatively, by using a wrapped fiber and decreasing the cable-lengthof the sensing channels, the spatial resolution can be increased whileretaining the same sensitivity as with a straight fiber.

The concepts described herein can be implemented using one, two, orthree fibers with different wrapping angles. Preferred embodiments ofthe sensing system include at least one fiber with a wrap angle of 90°,i.e. parallel to the cable axis, and one fiber with a wrap angle lessthan 45°. Still more preferred embodiments include a third fiber with awrap angle between 45° and 90°. Fibers with different wrapping angleshave different directional sensitivity, and by comparing their responsesone can determine the direction of wave propagation with respect to thefiber axis.

In addition, multiple fibers can be wrapped inside a single cable atdifferent radii. Alternatively, multiple cables each having a singlehelically wrapped fiber can be used. Still further, while certainembodiments are disclosed in terms of a fiber that is wrapped around acylindrical body, it will be understood that the fiber need not actuallyencircle the body but may instead change or reverse direction so as todefine fiber segments having a predetermined wrap angle alternating withbends or reversing segments.

Thus, as shown in FIG. 3, a particular preferred embodiment comprises aninner liner 15, a first sheath layer 16, and a second sheath layer 17.Wrapped around liner 15 and covered by sheath layer 16 are a plurality(three, as illustrated) of optical fibers 18. Fibers 18 are preferablywrapped at a first wrap angle with respect to the plane normal to thecable axis. Similarly, a plurality (three, again, as illustrated) ofoptical fibers 19 are preferably wrapped around liner 16 and covered bysheath layer 17. Fibers 19 are preferably wrapped at a second wrap anglethat is different from the first wrap angle of fibers 18. In onepreferred embodiment, one of fibers 18 or 19 is straight, i.e. with awrap angle of 90° and the other is wrapped with a small wrap angle, i.e.a wrap angle less than 45° with respect to the plane normal to the cableaxis. The use of different wrap angles provides different directionalsensitivities from which, by comparing their responses, it is possibleto determine the direction of wave propagation with respect to the fiberaxis. It will be understood that additional fibers having additionalwrap angles can also be included.

By way of example only an optical sensing system may include a first,straight fiber, a second fiber with a wrap angle of 30° with respect tothe plane normal to the cable axis, and a third fiber with a wrap angleof between 30° and 90°. The fiber wrapped at 30° gives exactly 2 m offiber per 1 m of axial length and the third fiber allows forverification of data from other two fibers.

In embodiments such as that of FIG. 3, the sheath layers may beconstructed of polyamide polymers, e.g. Nylon 12, or other suitablematerials whose elastic impedance, do not differ significantly fromimpedance of the formation. Fibers 18 and 19 may be any suitable tightbuffered optical fibers such as are known in the art. The inside ofliner 15 may be empty or filled with fluids, such as ground water,formation water, gel, or other suitable fluids.

In case of a non-perfect coupling between the cable and formation, thefiber sensitivity has the formS=cos² θ sin² α+(A+B sin² θ)cos² αwhere A and B are constants whose values depend on the materialproperties of the cable and formation.

The choice of the cable material depends on the concrete purpose of theDAS system. For example, a relatively stiff cable with a Young's modulusof several GPa provides a low material contrast with the formation,which corresponds to A=0 and B=0.5 in the above equation. Such a cablehas better directional sensitivity than a more flexible cable. Usingsuch cables may be preferable in a borehole environment or if the cableis buried deep in the subsurface. In case of soft cables, the value of Acan be much larger than 1 The signal from such cables will have weakdependence on the wave propagation angle, but such cables will have ahigher overall sensitivity. This is important for trenched cables thatlie close to the surface, where the pressure of the incident wave isvery low.

The quantities A and B depend on the cable construction and the acousticproperties of the surrounding medium (Vp, Vs, density). In thenear-surface, these medium properties vary over time due to seasonablechanges, rain, etc. These variations produce misleading time-lapseeffects that tend to mask true time-lapse signals from reservoirprocesses. One way to overcome this problem is to measure the seismicsignals along the cable as a function of incidence angle and determinethe quantities A and B ab initio. Such an approach may not be practicalin general, but may be feasible in time-lapse applications, i.e, onlydetermining the changes in A and B, information which would be useful toincrease the fidelity of time-lapse processing.

A fiber wrapped around a circular cylinder, however, does notdiscriminate between waves propagating normally to the cable axis fromdifferent azimuthal directions Azimuthal sensitivity can be added byusing helixes of noncircular, e.g. elliptical, wrapping shapes, whichallow detection of all three components of the incident waves.

DAS Cable with Built-in Cable Trajectory Visualization (RTCM) Capability

In some embodiments, the cable could include a distributed strainsensing (DSS) fiber similar to the one used in real-time compactionmonitoring (RTCM) systems. In RTCM systems, an optical fiber is usedthat contains thousands of fiber Bragg gratings (FBGs) wrapped around atubular. Because the fiber is helically wrapped, the strain response canbe decomposed into different deformation modes (bending, ovalization,axial strain) and the outputs of the decomposition can be used toproduce a three-dimensional image of the tubular shape.

The same principle can be applied to the cable disclosed in this presentinvention so as to measure the hole trajectory from the strain on thecable, which is important to know in some seismic applications. In thisembodiment, an additional fiber containing FBGs can be embedded into thecable or one of the fibers used for DAS can contain FBGs with gratingwavelengths that are sufficiently different from the interrogationwavelength of the DAS system. The RTCM interrogation unit would recordthe strain on the cable after/during installation to measure the shapeof the cable and infer the trajectory (azimuth, depth, etc.) of thehole. From the trajectory of the hole, the location of the DAS channelscan be derived in space relative to the seismic source(s) at the surfaceand to the formation.

In addition, this cable can be used as a permanent monitor of thegeomechanics of the field. By monitoring the change in cable shape overtime, it is possible to measure the amount of surface subsidence causedby oil and gas production. This information, when combined with theseismic data from DAS can improve the understanding of the reservoirdynamics and the geomechanics of a producing field.

Deployment Methodology to Allow Low-Noise Recording and Virtual SourceSeismic

For surface seismic applications, the optical sensing cableincorporating the present concepts can be deployed in one or moretrenches on the earth's surface or the seabed, or inside asmall-diameter hole, or “data-hole,” in consolidated formations in thesubsurface or subsea. The latter deployment mode tends to providehigher-quality data with higher-frequency content and allows virtualsource seismic monitoring. It also reduces the environmental footprintof the sensing system.

Suitable small-diameter holes can be drilled using low-cost drillingtechniques, such as horizontal directional drilling (HDD) orwater-jetting. HDD and water-jet drilling ore often used for installinginfrastructure such as telecommunications, power cables, gas mains etc.The horizontal or deviated hole may run several tens or hundreds ofmeters below the surface and may be hundreds or even thousands of meterslong.

Once a data-hole has been drilled, there are various ways to install theoptical sensing system. One way is to push a tube containing the opticalsensing system into the hole, thereby using the drilling hose or tube,if it is still in the hole, as a guide to position the sensing tubeinside the hole. After the sensing tube is in place, the drilling hoseor tube may be removed from the hole.

In still other embodiments (not shown), the optical fiber(s) areintegrated in the wall of the high pressure-hose or tubes used by thedrilling system. In these embodiments, once the data-hole has beendrilled, the pressure-hose/tubes incorporating the sensing system areleft behind in the hole.

In other embodiments, the data-hole may include a surface exit. In thiscase, the sensing system can be pulled into the hole via the surfaceexit when the drill-string is being retrieved from the hole.

In some embodiments, the tubular containing the optical sensing system10 may have an outer coating comprising a swellable elastomer 20, asshown in FIG. 4. As illustrated in FIG. 5, the elastomer swells when incontact with formation water or other pre-selected liquid, therebyfixing the sensing tube against the inner wall 22 of the borehole andensuring good mechanical and acoustic coupling with the formation.Swellable elastomers are known in the art. Alternatively, the annulusbetween the sensing tube and the formation may be filled with fluid, agel or cement.

Referring now to FIG. 6, after the sensing tube has been put in place, aseparate sensing rod 30 may be installed inside the sensing tube 10.Sensing rod 30 may contain a plurality of straight, sinusoidal, and/orwrapped fibers 32. If wrapped (not shown), fibers 32 preferably have alarge pitch, i.e. a small wrap angle, e.g. less than 45° and morepreferably less than 30°. If desired, rod 30 may be centralized andfixed inside the sensing tube by means of a layer of swellable rubber,fluid, gel, cement, etc., as shown at 34 in FIG. 7.

Since a straight cable is sensitive only to one direction (along thecable), it allows a simple partitioning of the signal recorded on awrapped fiber into along-the-cable and across-the-cable components,assuming that both fibers (wrapped and straight) are made of the samematerial and embedded in the same medium. If they are of differentmaterials or in different parts of the cable (center vs. periphery),their overall sensitivities to external formation strain may bedifferent—i.e., they may have different A and B coefficients in theequation above. In that case, a straight cable may still help calibrateor constrain the partitioning of the wrapped fiber signal but thecalibration would not be through direct subtraction of thealong-the-cable component.

Cable Configuration

In order to enable a decomposition of a signal into three orthogonalcomponents, it is necessary to use at least three fibers that incombination provide sensitivity in all three directions. Referring nowto FIG. 8, an alternative construction for an optical sensing systemwith 3 fibers comprises a cable 40 having a triangular cross-section andat least two orthogonal sinusoidal optical fibers 42, 44 and a straightfiber 41 therein.

An advantage of the triangular cross-section is that the cable has aflat bottom surface 43, which can be fixedly oriented with respect to,e.g., the inner or outer wall of a tubular, which in turn facilitatesazimuthal sensing. It will be understood that while cable 40 is shownwith a triangular cross-section, any polygon would be suitable. Further,if a flat bottom surface is not desired, the cable cross-section may beround, elliptical, oval, or any other shape. In order to facilitateinstallation of the cable with a known orientation, bottom surface 43,or all or a portion of one of the other surfaces may be color-coded orotherwise visually marked. In the absence of such external indicator,the determination of the azimuthal orientation of the cable must be madethrough first-arrival analysis.

Still referring to FIG. 8, fiber 42 will be sensitive to signals havingcomponents in the inline (x) and vertical (z) directions. Similarly,fiber 44 will be sensitive in the inline (x) and cross-line (y)directions. Fiber 41 has sensitivity in the inline (x) direction. Thethree fibers are assumed to be identically coupled to the formation.Accordingly, a combination of the responses of fibers 41, 42 and 44enables a decomposition of the signal into the x, y, and z directions.In other embodiments (not shown), a sinusoidal fiber may be disposedalong one, two, three, or more surfaces of a body having a polygonalcross-section. Thus, for example, three sinusoidal fibers may bedisposed, each against one side of a body having a triangularcross-section. Signals from those three fibers could also be decomposedto three orthogonal sets. Such cable may be easier to manufacture.

In another variation, the embodiment shown in FIGS. 9 and 10 comprises acable 46 having a hemi-circular cross-section and a flat bottom surface47. In cable 46, one fiber 48 is helically wrapped with a small wrapangle and a second fiber 49 is sinusoidal. Fiber 48 will be sensitive inthe inline (x), cross-line (y), and vertical (z) directions.

In either case, additional straight fibers 41 (shown in phantom in FIGS.8 and 9) can be included in the cable, as discussed above. Straightfiber 41 will be sensitive to the inline direction (x). By using acombination of horizontal, vertical, and straight fibers, preferablyrecording in the same conditions, it is possible to generate 3C data.

Deployment of a Multi-Component Cable

Referring now to FIGS. 11 to 13, a multi-component cable 50 inaccordance with a preferred embodiment comprises an inner tube 51, anexpandable layer 52 surrounding tube 51, and an expandable tube 60surrounding layer 52. Inner tube 51 is preferably substantially rigidand may comprise steel, polyamide, or the like. Inner tube 51 may befilled with a gel such as is known in the art or may be made solid usingpolyamide or the like. Layer 52 is preferably made of water- oroil-swellable elastomers, such as are known in the art. Tube 60 ispreferably constructed from a deformable material such as an elastomer.

Elongate sensor pads or strips 62 preferably extend the entire length ofthe cable. The material of which tube 60 is made is preferable flexibleand elastomeric so as to enable it to respond to the expansion of theunderlying swellable layer 52. In preferred embodiments, the pads aremade of Nylon 11, which is preferably also used for encapsulating thefiber optic and hydraulic control lines. Nylon 11 has a crush resistancein excess of 100 tons per square inch and excellent abrasion resistance.In some embodiments, sensor pads 62 define all or the majority of theouter surface of multi-component cable 50, so that tube 60 is not or notsubstantially exposed to the borehole wall. In these embodiments,multi-component cable 50 may have a more or less square cross-section.It will be understood that there are a variety of configurations inwhich sensor pads and/or additional protective layers might be appliedto or near the outer surface of tube 60.

The purpose of tube 60 is to protect and hold the desired sensing fibers(described below) in position while running in-hole. The materialpreferably provides mechanical support but is flexible enough to bepushed like a balloon against the wall of the hole during installation(described below).

During installation of the cable, the annulus between 52 and 60 ispreferable empty and sealed, or filled with a fluid that will notactivate the expandable components.

One or more straight or substantially straight optical fibers 54 arehoused in tube 51 and one or more sinusoidal fibers 64 are providedwithin expandable tube 60. In preferred embodiments, each sinusoidalfiber 64 is embedded in or mounted on an elongated sensor pad or strip62 having a relative high Young's modulus, e.g. 500 to 5000 MPa. In someembodiments, an optional straight fiber 57 (shown in FIG. 11 only) isincluded with each sinusoidal fiber 64 in or on one or more of thesensor pads 62.

In some preferred embodiments, as illustrated, there are four sensorpads 62 evenly azimuthally spaced around tube 60. This embodimentprovides two sets of fibers that are sensitive in two perpendicularplains. Having two sets of fibers for each orientation provides a usefulredundancy in case, for example, sensor pads 62 twist when pressedagainst the uneven wall of an imperfectly shaped borehole. Optionally,the sinusoidal fiber 64 in one pad may have a different period, similarto a different “wrap angle,” than that on the opposite pad. Thus, themulti-wrap-angle concepts discussed above could be used in conjunctionwith this cable design.

Sinusoidal fibers 64 and optional straight fibers 57 may be embedded insensor pads 62 in an extrusion process. Sensor pads 62 may comprise anysuitable extrusion materials such as are known in the art, includingpolyamide polymers, metal, or ceramic.

Turning now to FIGS. 14 and 15, multi-component cable 50 is shown in aborehole or data-hole 72 that has been drilled or otherwise provided inthe subsurface 70. Multi-component cable 50 can be pushed or pulled intothe data hole by any suitable means. As mentioned above, the annularspace between inner tube 51 and the tube 60 is kept sealed duringinstallation so as to prevent the swellable material 52 from activating.

Once the multi-component cable 50 is in place, swellable material(s) oflayer 52 can be caused to swell by pumping an appropriate fluid (e.g.water) through the annulus between layer 52 and tube 60. As shown inFIG. 15, when swelling is complete, layer 52 and tube 60 will occupy theentire space between inner tube 51 and the hole wall. Inner tube 51 willbe substantially centered in the hole and sinusoidal fibers 64 will beplaced in proximity to the hole wall. In preferred embodiments, layer52, tube 60 and sensor pads 62 are configured such that when swelling iscomplete, sensor pads 62 are pressed against the inside wall of thedata-hole 72.

Multi-component cable 50 provides 3C operability with good acousticcoupling to the formation. Further, since it can be used in asmall-diameter data-hole, multi-component cable 50 allows for relativelylow-cost deployment and greatly reduced HSE footprint.

While FIGS. 14 and 15 describe multi-component cable 50 with respect toa data-hole, it will be understood that cable 50 is equally useful forsurface applications. By way of example only, the embodiment shown inFIG. 16 comprises an optical sensing system 80 configured for use on theearth's surface. Like multi-component cable 50, optical sensing system80 includes an inner tube 51 that houses one or more optical fibers 54.In addition, tube 51 may house one or more communication or powertransmission lines 55. Alternatively, the optical fibers and theelectrical wires may be in separate tubes (not shown). Surrounding tube51 is an elongate body 82 having at least one flat bottom surface 83 anda top surface 84 that may be rounded. Also like multi-component cable50, optical sensing system 80 includes at least one, and preferably aplurality, of sensor pads 62 that each include at least one sinusoidalfiber 64. Pads 62 are preferably arranged so that fibers 64 aresensitive to signals that are normal to the axis of the system 80. As inthe embodiment illustrated in FIG. 16, one pad is preferably placedadjacent to bottom surface 83. In other preferred embodiments, at leastone pad is substantially vertical.

Body 82 is preferably constructed from a material having a Young'smodulus, similar to or higher than the Young's Modulus of the sensor pad62 or materials similar to the encapsulation materials used for fiberoptic and hydraulic downhole control lines, as are known in the art, soas to provide crush- and abrasion-resistance.

In preferred embodiments, system 80 is used on the earth's surface formonitoring seismic signals travelling through the subsurface. Thus, itmay be used in conjunction with a ground anchor 90 such as is shown inFIG. 17. Anchor 90 preferably includes arms 92 through which suitablefasteners can be used to anchor the system. Similarly, if it is desiredto affix system 80 to a curved surface, arms 92 can be curved as shownin FIG. 18. System 80 can be used for downhole or pipeline sensing inconjunction with an anchor and/or adhesive or other fastening means.

In addition to the various applications mentioned above, the opticalsensing systems described herein can be used as towed streamer cables ordeployed on the seabed (OBC). It is expected that DAS systems inunderwater applications would work better than in trenched cables onland because of the absence of surface waves subsea and because thesubsea acoustic media are stable over time and are not affected byseasonal changes.

The embodiments described herein can be used advantageously alone or incombination with each other and/or with other fiber optic concepts. Themethods and apparatus described herein can be used to measure arrivaltimes and waveforms of acoustic signals and in particular broadsideacoustic waves. Arrival times and waveforms give information about theformation and can be used in various seismic techniques.

In still other applications, the methods and apparatus described hereincan be used to detect microseisms and the data collected using thepresent invention, including broadside wave signals, can be used inmicroseismic localization. In these embodiments, the data are used togenerate coordinates of a microseism. In still other applications,ability of the present systems to detect broadside waves and axial wavesdistinguishably can be used in various DAS applications, including butnot limited to intruder detection, monitoring of traffic, pipelines, orother environments, and monitoring of various conditions in a borehole,including fluid inflow.

While preferred embodiments have been disclosed and described, it willbe understood that various modifications can be made thereto withoutdeparting from the scope of the invention as set out in the claims thatfollow.

We claim:
 1. A method of distributed acoustic sensing, the methodcomprising the steps of: providing a fiber optic distributed acousticsensing system, the system comprising a cable having a cable length, thecable comprising an elongated body having an outer surface, at least onestraight optical fiber extending parallel to a longitudinal axis of thecable along the cable length, and at least one helically wrapped opticalfiber extending along the cable length and having a first predeterminedwrap angle; transmitting optical signals into each of the opticalfibers; receiving backscattered signals out of each of the opticalfibers consisting of a component of said optical signals which componenthas been backscattered from impurities or inhomogeneities in each of theoptical fibers; observing changes in the backscattered signals caused byaxial stretching and compressing of each of the optical fibers caused byan incident wave; comparing the backscattered signals of the at leastone straight optical fiber and the at least one helically wrappedoptical fiber; and determining, based on the comparing of thebackscattered signals, a direction of wave propagation of the incidentwave with respect to said longitudinal axis of the cable for detectingbroadside waves and axial waves distinguishably.
 2. The method accordingto claim 1, further comprising reconstructing a seismic wave amplitudefrom the backscattered signals.
 3. The method according to claim 1,wherein the incident wave includes acoustic signals travelling in aformation induced by seismic or microseismic events.
 4. The methodaccording to claim 3, comprising the step of arranging the cable on asurface of the earth.
 5. The method according to claim 3, comprising thestep of arranging the cable within the formation.
 6. The methodaccording to claim 3, comprising the step of arranging the cable withina borehole in the formation.
 7. The method according to claim 1, whereinthe first predetermined wrap angle is measured with respect to a planenormal to the axis of the elongated body and wherein the firstpredetermined wrap angle is smaller than 90°.
 8. The method according toclaim 1, further including providing a first sheath layer on the outsideof the elongated body and covering the elongated body and the firststraight optical fiber.
 9. The method according to claim 8, the cableincluding at least a second helically wrapped optical fiber wrappedaround the outside of the first sheath layer.
 10. The method accordingto claim 9, wherein the first helically wrapped optical fiber and thesecond helically wrapped optical fiber define different wrap angles. 11.The method according to claim 10, wherein the wrap angles are measuredwith respect to a plane normal to the axis of the elongated body andwherein the first predetermined wrap angle is 90° and the secondpredetermined wrap angle is less than 45°, or wherein the secondpredetermined wrap angle is 90° and the first predetermined wrap angleis less than 45°.
 12. The method according to claim 11, the cablefurther including a third helically wrapped optical fiber disposed onthe outer surface at a wrap angle between 90° and 45°.
 13. The method ofclaim 1, comprising the step of measuring seismic signals along thecable as a function of incidence angle of the seismic signals.
 14. Themethod of claim 1, the step of comparing the backscattered signals ofthe at least one straight optical fiber and the at least one helicallywrapped optical fiber comprising using the backscattered signals of theat least one straight optical fiber to calibrate partitioning of thebackscattered signals of the at least one helically wrapped opticalfiber.
 15. The method of claim 1, the at least one straight opticalfiber being arranged in the cable.
 16. The method according to claim 1,comprising the steps of towing the cable and using the cable as towedstreamer cable.
 17. The method according to claim 1, comprising the stepof measuring arrival times and waveforms of acoustic signals, theincident wave comprising the acoustic signals.
 18. A method ofdistributed acoustic sensing, the method comprising the steps of:providing a fiber optic distributed acoustic sensing system, the systemcomprising a cable having a cable length, the cable comprising anelongated body having an outer surface, at least one straight opticalfiber extending parallel to a longitudinal axis of the cable along thecable length, and at least two orthogonal sinusoidal optical fibersextending along the cable length; transmitting optical signals into eachof the optical fibers; receiving backscattered signals out of each ofthe optical fibers consisting of a component of said optical signalswhich component has been backscattered from impurities orinhomogeneities in each of the optical fibers; observing changes in thebackscattered signals caused by axial stretching and compressing of eachof the optical fibers caused by an incident wave; comparing thebackscattered signals of the at least one straight optical fiber and theat least two orthogonal sinusoidal optical fibers; and determining,based on the comparing of the backscattered signals, a direction of wavepropagation of the incident wave with respect to said longitudinal axisof the cable for detecting broadside waves and axial wavesdistinguishably.